The present invention relates to position sensing devices, and more particularly relates to techniques for exciting such devices.
In order to produce output signals that accurately represent position, it is necessary to maximize the detected output signal. Because the excitation signal typically crosses an air gap between sensor plates, it is desirable to use a relatively high frequency, large magnitude excitation signal. However, if the high powered excitation signal must travel a long distance to the capacitive sensor, the excitation signal can be degraded or can interfere with the operation of other components.
Although the excitation signal ideally would be generated as close to the capacitive sensor as possible in order to minimize these types of problems, previously available devices used remotely generated excitation signals. These devices did not have the capacity to generate excitation signals which were high in frequency, large in amplitude and close to the capacitive position sensor. Although it was possible to use transformers to proximately generate these types of signals, transformers are very heavy components that consume large amounts of power.
A need has long existed in the industry for a capacitive position sensor excitation technique which provides a high frequency, high amplitude excitation signal with reduced cost, weight, size and power consumption.
The present invention is useful in a position sensor for determining relative changes in position of a first member movable with respect to a second member. According to a preferred apparatus aspect of the invention, the sensor includes at least a first conductor movable with one of the first and second members, as well as a second conductor displaced from the first conductor by a gap. The second conductor is energized in response to an electrical first signal so that at least a portion of the first signal is coupled to the first conductor across the gap. The coupling induces a current in the first conductor having a current value which varies in response to relative changes in position of the first and second members. A power supply furnishes a predetermined supply voltage to the sensor. A pulse generator generates output pulses at a predetermined repetition rate, and transmits the pulses to a resonant circuit having a center frequency within a range of resonant frequencies that includes the repetition rate of the pulse generator. The resonant circuit is responsive to the output pulses in order to generate the first signal with a voltage greater than the supply voltage so that the current value is sufficiently large to facilitate further processing.
According to a preferred method aspect of the invention, a predetermined supply voltage is furnished to the sensor and output pulses are generated at a predetermined repetition rate. The first signal is generated with a signal voltage greater than the supply voltage so that the current value is sufficiently large to facilitate processing.
By using the foregoing techniques, an excitation signal with adequate amplitude and frequency can be generated while reducing weight, cost and power consumption. The invention enables an excitation signal to be produced adjacent the conductor which is energized on one side of the gap, thereby avoiding long coaxial cables which otherwise would need to be used. In addition, the use of the resonant circuit avoids the need for heavy, large and expensive transformers which have been required in the past.